Flywheel energy storage devices and systems are known for storing energy and releasing stored energy on demand. Known flywheel assemblies have a traditional rotor design sometimes made with carbon fiber composites. Such rotors have a shaft on which the motor/generator (M/G) and bearing permanent magnets (PMs) are mounted. The shaft is conventionally connected to the rim via a hub. The shaft-and-hub flywheel design is limited in terms of its achievable upper-end velocity. Matching useable materials for components in the flywheel assembly has been problematic since the radial growth and dimension of the components varies as the rotor velocity increases. The hub must mechanically couple the shaft to the rim without introducing bending modes into the rotor structure through the range of operating frequencies in the operating speed range of the flywheel. However, the shaft often exhibits negligible radial growth while the rim exhibits significant radial growth.
Therefore, the higher speeds for flywheels enabled by the use of ever-advancing materials unfortunately exacerbates the growth-matching problem for the flywheel components as the increased radial growth of the rotor outpaces any growth exhibited by other connected components such as, for example, the connecting shaft, and/or other rotating components attached to the rotor, such as, for example, the permanent magnets (PMs). Further, the overall efficiency afforded by flywheel technology is limited by the presently available materials that often fail when the flywheel is run at speeds that exceed material tolerances.
In addition, higher rotational speeds desired by present and next-generation flywheels will cause the premature failure and otherwise inhibit optimal performance of certain component parts in the flywheel assembly. One significant area of concern is the magnets that are critical to the flywheel operation. Ceramic-type magnets have been used in flywheel assemblies. However, such magnets have not been practical at higher rotational speeds due to their inherent characteristics including, but not limited to, their brittleness, for example. Therefore, as rotational flywheel speeds increase, various magnet types are needed. Known ceramic magnets are generally limited to circumferential velocities of less than about 300 m/s. Magnets having desirable properties, including their ability to expand as the rotor material itself expands in operation at very high speeds would be desirable. However, such magnet sheets can be too pliable, leading to a condition known as “creep” or “flow” whereby the dimension of the magnet sheet can change dimension unpredictably and non-uniformly, and undesirably and unpredictably extrude and/or extend beyond the dimension of the rotor in an uncontrolled manner. Such creep depends on the material properties of the magnet sheets, but it is likely to occur at circumferential velocities that exceed about 500 m/s.